Children with nonorganic failure to thrive present a difficult challenge to the pediatrician as there is no specific treatment once organic etiologies have been ruled out. This study is investigating whether Cyproheptadine HCL (Periactin), an antihistamine with appetite stimulating effects, will promote weight gain in children with nonorganic failure to thrive. Weight gain in these children will be evaluated to determine if it is due to a physiological effect of the medication or to an improvement in psychsocial functioning in the family.